1. Field of the Invention
The present invention relates generally to a high frequency wiring board provided with a high frequency transmission line having a signal conductor line and a grounding layer formed parallel to the signal conductor line through a dialectic board, and its connecting structure. More particularly, the present invention relates to a high frequency wiring board suitable for a semiconductor device containing package, a multi-layer wiring board, or the like which carries a high frequency semiconductor device in a millimeter wave region having a frequency of not less than 30 GHz (and further, not less than 50 GHz) and its connecting structure.
2. Description of Related Art
As radio waves used for information transmission, radio waves in a microwave region having a frequency of 1 to 30 GHz have been conventionally used. In recent years, we have entered upon the sophisticated information year. Therefore, we have examined the use of a millimeter wave region having a frequency of 30 to 300 GHz. For example, application systems using radio waves in a millimeter wave region, for example, a high speed radio data communication system (radio LAN (Local Area Network)) in an office have been also proposed.
As a wiring board such as a package containing or carrying a high frequency semiconductor device (hereinafter merely referred to as a high frequency device) used for such application systems, a so-called metal package has been conventionally used. The metal package has a structure in which a connecting board made of ceramics is joined to a metal frame in order to minimize the transmission loss of a high frequency signal.
FIG. 16A is a plan view showing a mounting structure in which a high frequency device is contained in a conventional metal package and is mounted on an external circuit board, and FIG. 16B is a cross-sectional view of the mounting structure. In FIG. 16A, a cover is omitted.
A metal package 33 comprises a metal board 31 and a cover 32. A connecting board 36 in which a signal conductor line 35 is formed on a ceramic board 34 is mounted on a part of the metal package 33. The signal conductor line 35 is electrically connected to a high frequency device 37 (indicated by hatching) carried in the metal package 33 by a ribbon or the like.
The metal package 33 is fixed to the surface of a base board 38 (indicated by hatching) by screws 39 or the like. A circuit board 42 in which a signal conductor line 41 is formed on the surface of a dielectric board 40 is provided on the surface of the base board 38. The signal conductor line 41 is electrically connected to the signal conductor line 35 in the connecting board 36 by a ribbon, a wire, or the like.
The assembly of such a metal package is complicated. Accordingly, there have been difficulties in mass production cost reduction at the time of fabricating a module have been a problem.
In order to solve this problem, it has been proposed that a signal conductor line is pulled out to the reverse surface of the package using a through hole conductor or the like from the inside of the dielectric board. That is, a connecting terminal portion is formed at a terminal end of the signal conductor line in the form of a through hole. The package is soldered to and surface-mounted on a high frequency circuit formed on the surface of another dielectric board by soldering reflow.
FIGS. 17, 18A, 18B, and 18C are illustrations for explaining the structure of a high frequency package using such a through hole conductor. As illustrated in the schematic cross-sectional view of FIG. 17, a dielectric board 51 and a cover 52 form a cavity in a high frequency package 50, and a high frequency device 53 is contained in the cavity. A signal conductor line 54 having its one end connected to the high frequency device 53 by a ribbon or the like is formed on the surface of the dielectric board 51. A grounding layer 55 (indicated by hatching) having a pattern shown in FIG. 18A is formed inside the dielectric board 51.
The other end of the signal conductor line 54 is connected to a through hole conductor 56 formed upon penetrating the dielectric board 51 so as not to come into contact with the grounding layer 55. The through hole conductor 56 is connected to a signal conductor line 57 formed on the reverse surface of the dielectric board 51. Consequently, the signal conductor line 54 is electrically connected to the signal conductor line 57 through the through hole conductor 56.
A pair of connecting grounding conductors 58 (indicated by hatching) is provided with spacing on both sides of an end of the signal conductor line 57 (indicated by hatching), as shown in FIG. 18B, on the reverse surface of the dielectric board 51. The grounding conductors 58 are electrically connected to the grounding layer 55 inside the dielectric board 51, respectively, by viahole conductors 59.
It is considered that when the viahole conductors 59 are formed near an end side surface a50 of the dielectric board 51, a portion between the viahole conductor 59 and the dielectric board 51 is liable to be cracked, for example. Generally, the viahole conductor 59 is formed at a position spaced not less than 2 mm apart from the end side surface a50 of the dielectric board 51 from the viewpoint of processability of a viahole by a micro-drill or the like.
The spacing between the viahole conductor 59 and the signal conductor line 57 is generally set to not less than 1 mm such that solders are not brought into contact with each other and short-circuited by making connection using the solders. That is, the spacing is approximately 0.5 times the signal wavelength of a high frequency signal in the dielectric board 51 composed of alumina (having a dielectric constant of 8.9) ceramics, for example, when a high frequency signal having a frequency of 50 GHz is transmitted. In this construction, however, the transmission loss of the high frequency signal is large from the following reasons, and the signal cannot, in some cases, be transmitted.
In the above-mentioned construction, a connecting terminal portion is constituted by the dielectric board 51, the signal conductor line 57, the grounding layer 55, the pair of connecting grounding conductors 58, and the viahole conductor 59. The connecting terminal portion is generally designed such that the impedance, in a cross section perpendicular to the direction of signal transmission, of the connecting terminal portion coincides with the impedance, in a cross section perpendicular to the direction of signal transmission, of a high frequency transmission line in order to diminish the reflection of the high frequency signal from the high frequency transmission line. In this case, however, the transmission loss of the high frequency signal is large from the following reasons, and the signal cannot, in some cases, be transmitted.
On the other hand, a grounding layer 61 is formed, as shown in FIG. 17, inside an external circuit board 60 on which the package 50 is mounted. A signal conductor line 62 (indicated by hatching) is formed, as shown in FIG. 18C, on the surface of the external circuit board 60. In a connecting portion with the package 50, connecting grounding conductors 63 (indicated by hatching) are formed with spacing on both sides of the signal conductor line 62. The connecting grounding conductors 63 are electrically connected to each other by a grounding layer 61 and viahole conductors 64.
The package 50 is mounted on the surface of the external circuit board 60 by electrically connecting the signal conductor lines 57 and 62 and electrically connecting the connecting grounding conductors 58 and 63, respectively, using solders 65 such as solder.
The package 50 can be thus collectively connected mechanically and electrically to the external circuit board 60 by reflow or the like. Consequently, the package 50 is more advantageous in that it is possible to improve quantity production and lower cost at the time of module fabrication, as compared with the metal package 33 shown in FIGS. 16A and 16B.
In the structure of the package 50 shown in FIG. 17, however, high frequency signal transmission characteristics in the connection terminal portion comprising the signal conductor line 57 formed on the reverse surface of the dielectric board 51 and the pair of connecting grounding conductors 58 formed on both sides thereof are not necessarily sufficient. That is, the connecting terminal portion has good transmission characteristics when the signal to be transmitted is a microwave signal having a frequency of less than 30 GHz (particularly not more than 3 GHz). When the frequency of the signal to be transmitted is very high, that is, not less than 30 GHz (particularly, not less than 50 GHz) in a millimeter wave region, however, it is difficult to transmit the signal between the package and the external circuit board. That is, in a mounting structure in which the package 50 is mounted on the external circuit board, the transmission loss of the high frequency signal to e transmitted therebetween is large. The transmission itself of the signal may, in some cases, be difficult.
Various considerations of the cause prove that the transmission characteristics are degraded because resonance occurs between the viahole conductors 59 and the end side surface a50 of the dielectric board 51. When the signal frequency is low, no resonance occurs because the signal wavelength is large. Contrary to this, as the frequency increases, and the signal wavelength decreases, resonance may easily occur.
A grounding current playing a part in signal transmission in a high frequency wiring board flows with the grounding current concentrated on the grounding layer just below the signal conductor line. In the connecting terminal portion on the package 50, a signal current in the signal conductor line 57 directly flows through the signal conductor line 62 on the external circuit board 60 via the solder 65 such as solder. Contrary to this, the grounding current flowing through the grounding layer 55 in the package 50 branches into the pair of viahole conductors 59 in the connecting terminal portion, and is introduced into the grounding layer 61 via the solder 65 and the viahole conductors 64 on the external circuit board 60, to reach a portion just below the signal conductor line 62 in the external circuit board 60. Therefore, the path length of the grounding current is larger than the path length of the signal current.
In a case where the difference in the path length is 1 mm, for example, when the frequency of the signal is not more than 50 GHz, for example, 3 GHz, the difference in the path length is not more than 0.033 times the wavelength of the signal in alumina ceramics. Accordingly, there is little phase difference between the signal conductor line and the grounding layer. Contrary to this, when the signal is a high frequency signal having a frequency of not less than 50 GHz, the difference in the path length cannot be ignored with respect to the wavelength of the signal. That is, a large phase difference occurs between the signal conductor line and the grounding layer. The signal is reflected by the phase difference, increasing the transmission loss of the signal.
Furthermore, as the frequency of the signal which is transmitted through the high frequency transmission line greatly increases to not less than 30 GHz, for example, the wavelength thereof decreases. Therefore, a quarter-wavelength representing sensitivity to a structural change of the high frequency signal approximates to the size of a constituent element of the high frequency transmission line, for example, the thickness of the dielectric board. In the connecting portion having a surface mounting structure shown in FIGS. 17, 18A to 18C, therefore, coupling between the signal conductor line on the package 50 and the ground occurs between the signal conductor line and not only the ground portion having a cross section, perpendicular to the signal transmission direction of the connecting portion but also the grounding layer 55 in the high frequency transmission line and the grounding layer 61 in the external circuit board 60. Consequently, the actual three-dimensional impedance of the connecting portion is smaller than the two-dimensional impedance, in a cross section perpendicular to the direction of signal transmission, of the connecting portion.
In other words, in a portion where the structure of the high frequency transmission line is changed, for example, the connecting portion having a surface mounting structure, when the signal frequency increases, a stray capacitance is created. When the impedance, in the cross section perpendicular to the direction of signal transmission, of the connecting portion is matched with the impedance of the high frequency transmission line in accordance with the conventional idea, the actual impedance of the connecting portion for the high frequency signal is smaller than the impedance of the high frequency transmission line. The signal is reflected by the impedance mismatching, increasing the transmission loss of the signal.
An object of the present invention is to provide a high frequency wiring board having a dielectric board provided with a high frequency transmission line comprised of a signal conductor line and a grounding layer, the high frequency wiring board being capable of reducing, when connected to another high frequency circuit, the transmission loss of a high frequency signal in a connecting portion and a connecting structure of such high frequency wiring board.
A high frequency wiring board according to the present invention comprises a dielectric board; a high frequency transmission line, having a signal conductor line formed on the surface of the dielectric board and having its terminal end extending toward the vicinity of an end side surface of the dielectric board and a grounding layer formed inside or on the reverse surface of the dielectric board parallel to the signal conductor line, for transmitting a high frequency signal; and a connecting terminal portion of a transmission loss reducing structure, provided at a terminal end of the high frequency transmission line, in order to make connection with another high frequency circuit, the connecting terminal portion including connecting grounding conductors formed with spacing on both sides of the signal conductor line on the surface of the dielectric board and through conductors provided upon penetrating the dielectric board for connecting the connecting grounding conductors and the grounding layer.
It is preferable that the distance between the through conductors and the end side surface of the dielectric board is not more than 0.3 times the signal wavelength, in the dielectric board, of the high frequency signal.
By this construction, it is possible to prevent resonance from occurring between the through conductors and the end side surface of the dielectric board. As a result, the loss of the high frequency signal in the entire connecting portion with the other external circuit is reduced, thereby making it possible to satisfactorily transmit the high frequency signal.
In the present invention, the connecting terminal portion on the wiring board is constituted by a coplanar line having a pair of connecting grounding conductors formed on both sides of the signal conductor line. Therefore, the connection with the other external circuit can be achieved by connecting the coplanar lines. This also allows the reflection of the high frequency signal to be diminished.
It is preferable that the distance between the through conductors and the end side surface of the dielectric board is not less than 0.05 mm.
A side surface of the through conductor may be exposed from the end side surface of the dielectric board. By this construction, a dielectric is not interposed between the through conductors and the end side surface of the dielectric board. Accordingly, it is possible to prevent resonance from occurring between the through conductors and the end side surface of the dielectric board. As a result, the loss of the high frequency signal in the entire connecting portion with the other external circuit is reduced, thereby making it possible to satisfactorily transmit the high frequency signal.
The high frequency transmission line may be one through which a high frequency signal having a frequency of not less than 30 GHz is transmitted.
It is preferable that the line width, in the connecting terminal portion, of the signal conductor line is smaller than the line width, in the high frequency transmission line, of the signal conductor line. Consequently, it is possible to weaken coupling between the signal conductor line and the grounding layer to relatively strengthen coupling between the signal conductor line and the connecting grounding conductors. That is, it is possible to convert an electromagnetic field distribution in the connecting terminal portion into a distribution closer to an electromagnetic field in the coplanar line, thereby making it possible to diminish the reflection of the signal by the change in the electromagnetic field distribution.
From the same reason, it is preferable that the grounding layer positioned between at least the pairs of through conductors in the connecting terminal portion on the high frequency wiring board and facing to the signal conductor line is taken as a non-grounding region. Consequently, it is possible to convert an electromagnetic field distribution in the connecting terminal portion into a distribution closer to an electromagnetic field in the coplanar line, thereby making it possible to diminish the reflection of the signal. This is effective in making it possible to transmit the high frequency signal with low loss.
The connecting terminal portion may be connected to the other high frequency circuit by a solder.
It is preferable that the distance between the center of the signal conductor line and the through conductors is not more than 0.25 xcexg (particularly, not more than 0.15 xcexg) (xcexg: the signal wavelength, in the dielectric board, of the high frequency signal). In this case, the high frequency transmission line may be one through which a high frequency signal having a frequency of not less than 50 GHz is transmitted.
By this construction, in a case where the high frequency signal having a frequency of not less than 50 GHz is transmitted, a grounding current in the grounding layer just below the signal conductor line playing a part in the transmission of the high frequency signal is transmitted to the connecting grounding conductor with a small phase difference from a current in the signal conductor line. Therefore, the reflection of the high frequency signal in the connecting terminal portion is diminished, thereby making it possible to satisfactorily transmit the high frequency signal. As a result, the phase difference in a connecting portion with the other high frequency circuit is reduced, and the reflection of the high frequency signal in the entire connecting portion with the other external circuit is diminished, thereby making it possible to satisfactorily transmit the high frequency signal.
The through conductor may be a viahole conductor formed by filling a viahole formed upon penetrating the dielectric board with metal paste and sintering the metal paste.
It is preferable, from a view point to increase the reliability of the through conductor, that the maximum diameter of a cross section of the through conductor is not more than 0.5 mm.
It is preferable that letting Z1 be the impedance of the connecting terminal portion, and letting Z0 be the impedance of the high frequency transmission line, a relationship of 1.4 Z0xe2x89xa6Z1xe2x89xa61.8 Z0 is satisfied. In this case, letting W1 be the line width, in the connecting terminal portion, of the signal conductor line, and letting W0 be the line width, in the high frequency transmission line, of the signal conductor line, a relationship of 0.4 W0xe2x89xa6W1xe2x89xa60.8 W0 is satisfied.
By this construction, even when the signal having a frequency of not less than 30 GHz is transmitted, the actual impedance of the connecting portion for the high frequency signal and the impedance of the high frequency transmission line can be matched with each other. Consequently, it is possible to reduce the transmission loss of the high frequency signal in the connecting portion with the external circuit.
A connecting structure of the high frequency wiring board according to the present invention is obtained by connecting the first high frequency wiring board and the second high frequency wiring board at least one of which has (preferably both of which have) the above-mentioned characteristics, wherein the respective signal conductor lines and the respective pairs of connecting grounding conductors in the connecting terminal portions on the first and second high frequency wiring boards are respectively connected to each other through solders. It is preferable that the respective connecting terminal portions on the first and second high frequency wiring boards have the same structural features.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.